A common standard for multimedia communications over circuit switched networks like ISDN (Integrated Services Digital Network) is the H.320 standard from the International Telecommunications Union (ITU). H.320 is an umbrella standard with several sub-standards defining the different protocol layers. An example of a sub-standard is H.221. H.221 is the framing protocol of H.320. The purpose of H.221 is to define a frame structure for audiovisual teleservices.
Video conferencing calls (hereafter referred to as H.320 calls) over ISDN networks need more bandwidth than normal voice telephony calls. The protocol ISO 13871, normally referred to as BONDING, supports communication over a set of separate ISDN channels to combine their bandwidths into a single coherent channel, hereafter referred to as an H.320 link. For example, a 256 kb/s data stream can be supported over four 64 kb/s channels. Sound and picture quality correlates with the number of channels (bandwidth) used.
BONDING (ISO 13871) is a technique first drafted by the BONDING consortium, and later maintained by the International Organization for Standardization (ISO). The technique is used to create frame structures and procedures for establishing a wideband communication connection by combining multiple switched 56/64 kbit/s channels. BONDING was originally described in the specification “Interoperability Requirements for Nx56/64 kbit/s Calls” issued by the BONDING consortium, the entirety of which is hereby incorporated by reference. The current protocol is described in the specification ISO 13871, “Digital Channel Aggregation,” maintained by the International Organization for Standardization, the entire contents of which are hereby incorporated by reference. The main task of a BONDING technique is to align data octets of the individual channels. Channels used for the wideband connection are routed independently of each other. Thus, the data in each channel might be individually delayed relative to the data in other channels. The defined frame structure for each 56/64 kbit/s bearer channel provides for the alignment of data octets from the individual channels to their original sequence before reforming the individual channels into a composite serial data stream at the terminating end.
H.320 calls (transmitting data over ISDN using bearer capability “unrestricted digital”) are exposed to more instability and disconnections on low quality (noisy) ISDN networks than normal voice telephony calls (transmitting voice over ISDN using bearer capability “speech”). Low quality ISDN network conditions can cause H.320 calls to disconnect frequently. This is because H.320 links are made of several separate ISDN channels combined by the BONDING process. As an example, when the timing of one channel slips, a resynchronization process is required for the whole link. This will appear to the user as a disturbing interruption in the communication with the other party. Further, a H.320 link sometimes has to re-route one of the B-channels. Then the BONDING procedure must be carried out again, and the user will experience another interruption. These interruptions may also cause complete disconnections.
Such communication breakdowns between the calling parties cause users to have bad experiences with video conferencing.